NSILA-s (nonsuppressible insulin-like activity soluble in acid ethanol) is a serum peptide that has insulin-like and growth promoting effects. We have demonstrated that this peptide binds with high affinity to its own receptors and low affinity to insulin receptors. In addition, this peptide has a relatively high affinity for liver plasma membrane insulinases and inhibits insulin degradation. About 90-95% of plasma NSILA-s is bound to a high molecular weight binding protein. Using a radioreceptor assay for NSILA-s we have found elevated levels in some patients with tumor hypoglycemia.